As the curtains parted, the frenzied car aficionados raced to get out their smartphones to capture the moment for posterity. Several of them cheered.

They had come to see Lot No. 362, the much-heralded Porsche Type 64, a swooping Nazi-era roadster that was built by the automaker Ferdinand Porsche nine years before he founded his car company.

The bidding on the avant-garde coupe, referred to by some car collectors as the “first” Porsche, had been expected to open at $13 million during an RM Sotheby’s Auction on Saturday night in Monterey, Calif. — until an auctioneer flubbed the numbers.

“When they mentioned 30 million to start, I thought that’s quite a strong starting price,” David Lee, a car collector and businessman from the Los Angeles area who was in the audience, said in an interview on Sunday. “The auctioneer had a British accent and didn’t say the teens well. Is he really saying 30 or 13?”

Displayed on a projection screen, the high bid of $30 million zoomed up to $40 million. Then it hit $50 million and $60 million, before crescendoing at $70 million. That would have been substantially higher than the record $48.4 million a 1962 Ferrari 250 GTO fetched last year at another Sotheby’s auction.

Then the auctioneer abruptly declared that the high bid was $17 million, not $70 million.

“Why would anybody dump 10 million increments like it’s nothing?” Mr. Lee said. “It’s just pretty unbelievable. He brought it down from 70 to 17 to save the situation.”

“As bidding opened on the Type 64, increments were mistakenly displayed on the screen, causing unfortunate confusion in the room,” RM Sotheby’s said in a statement on Sunday. “We take pride in conducting our world-class auctions with integrity and we take our responsibility to our clients very seriously.”

“This was in no way intentional on behalf of anyone at RM Sotheby’s, rather an unfortunate misunderstanding amplified by excitement in the room,” the auction house said.

Although the classic roadster received a high bid of $17 million, RM Sotheby’s said that did not meet the “reserve,” which is a minimum price set by the seller.

“It is difficult to put a price on such a unique and historically significant artifact and, despite interest from discerning collectors, we were unable to reach common ground between seller and buyer on the night,” RM Sotheby’s said.

Just three of the Porsche-designed Type 64s were planned for production, and only one remains. The car’s modified, air-cooled Volkswagen four-cylinder engine puts out about 32 horsepower and can reach a top speed of about 88 miles per hour.

The car was supposed to be part of a propaganda campaign, marking the Nazis’ 1938 alliance with Italy and the absorption of Austria. Porsche also designed the KdF-Wagen, commissioned by Hitler as the people’s car, which became famous after the war as the Volkswagen Beetle.

Chris Harris of the British series “Top Gear” fawned over the Porsche Type 64 during a recent test drive.

“This is like breaking into a safe somewhere in America,” he said. “Someone finds an old safe in a building, and you crack it open and you find a little book that says ‘This is the recipe for Coca-Cola.’” That what this is in a car.”

Mr. Lee said he had difficulty understanding the Sotheby’s auctioneer all weekend.

“I’ve never seen anything like that,” he said. “It’s really an amazing situation that we witnessed.”

RM Sotheby’s was still advertising the car on its auction website. Sotheby’s acquired an ownership interest in RM Auctions, which specializes in vintage cars, in early 2015.

“We will continue making every effort to sell the car,” the auction house said, adding, “This is the genesis and antecedent of Porsche’s historical evolution that would follow, and the oldest car to wear the Porsche name. Its design cues continue to influence those of modern Porsche models.”

Mr. Lee did not come away from the Monterey auctions empty-handed.

“I’m more of a Ferrari collector,” said Mr. Lee, who posted the winning bid on a 1967 Ferrari 330 GTS auctioned by Gooding & Company.